statquant statquant - 1 month ago 14
R Question

Change timezone in a POSIXct object

Playing with dateTimes and timezone can be tricky in R.
Here is my question: I want to change the time-zone on a

POSIXct
object

R) data <- data.frame(x=c(1,2),dateTime=as.POSIXct(c("2010-03-11 03:30:00.432","2010-03-15 03:30:00.432"),format="%Y-%m-%d %H:%M:%S",tz="America/Montreal"))
R) data
x dateTime
1 1 2010-03-11 03:30:00
2 2 2010-03-15 03:30:00
R) str(data)
'data.frame': 2 obs. of 2 variables:
$ x : num 1 2
$ dateTime: POSIXct, format: "2010-03-11 03:30:00" "2010-03-15 03:30:00"


But if I want to change the timezone, the only thing I found is:

R) data$dateTime2 = format(data$dateTime,tz="Europe/Paris")
R) str(data)
'data.frame': 2 obs. of 3 variables:
$ x : num 1 2
$ dateTime : POSIXct, format: "2010-03-11 03:30:00" "2010-03-15 03:30:00"
$ dateTime2: chr "2010-03-11 09:30:00" "2010-03-15 08:30:00"


Or

R) data$dateTime2 = as.POSIXlt(data$dateTime,tz="Europe/Paris")
R) str(data)
'data.frame': 2 obs. of 3 variables:
$ x : num 1 2
$ dateTime : POSIXct, format: "2010-03-11 03:30:00" "2010-03-15 03:30:00"
$ dateTime2: POSIXlt, format: "2010-03-11 09:30:00" "2010-03-15 08:30:00"


God (or somebody on SO) knows why it doesn't work with
POSIXct


R) data$dateTime2 = as.POSIXct(data$dateTime,tz="Europe/Paris")
R) str(data)
'data.frame': 2 obs. of 3 variables:
$ x : num 1 2
$ dateTime : POSIXct, format: "2010-03-11 03:30:00" "2010-03-15 03:30:00"
$ dateTime2: POSIXct, format: "2010-03-11 03:30:00" "2010-03-15 03:30:00"


Do I need to convert to
character
and cast back to
POSIXct
?

Answer

It doesn't work with POSIXct because base::as.POSIXct.default simply returns x if it's already POSIXct. You can change the timezone via the tzone attribute:

attr(data$dateTime, "tzone") <- "Europe/Paris"